Rammed earth, also called pisé, is a technique for constructing foundations, floors, and walls using compacted natural raw materials such as earth, chalk, lime, or gravel. It is an ancient method that has been revived recently as a sustainable building method. Pisé also refers to a material for sculptures, usually small and made in molds. It has been especially used in Central Asia and Tibetan art, and sometimes in China. Edifices formed of rammed earth are found worldwide, in a range of environments including temperate, wet, semiarid desert, montane, and tropical regions. The availability of suitable soil and a building design appropriate for local climatic conditions are two factors that make its use favorable.
Building process
Making rammed earth involves compacting a damp mixture of subsoil that has suitable proportions of sand, gravel, clay, silt, and stabilizer if any, into a formwork (an externally supported frame or mold). Historically, additives such as lime or animal blood were used to stabilize it. Soil mix is poured into the formwork to a depth of 10 to 25 cm (4 to 10 in) and then compacted to approximately 50% of its original volume. The soil is compacted iteratively in batches or courses so as to gradually erect the wall up to the top of the formwork. Tamping was historically manual with a long ramming pole by hand, but modern construction systems can employ pneumatically-powered tampers.
After a wall is complete, it is sufficiently strong to immediately remove the formwork. This is necessary if a surface texture is to be applied, e.g., by wire brushing, carving, or mold impression because the walls become too hard to work after approximately one hour. The compressive strength of rammed earth increases as it cures. Cement-stabilized rammed earth is cured for a minimum period of 28 days. In modern rammed-earth buildings, the walls are constructed on top of conventional footings or a reinforced-concrete slab base.
The construction of an entire wall begins with a temporary frame, the "formwork", which is usually made of wood or plywood, as a mold for each wall section's desired shape and dimensions. The form must be durable and well-braced, and the two opposing faces must be clamped together to prevent bulging or deformation caused by the large compressing forces. Formwork plays an important role in building rammed-earth walls. Historically, wooden planks tied using rope were used to build walls. Modern builders use plywood or steel (or both) to build formwork.
Characteristics
The compressive strength of rammed earth is dictated by factors such as soil type, particle size distribution, amount of compaction, moisture content of the mix and type/amount of stabiliser used. Well-produced cement-stabilised rammed-earth walls can be anywhere between 5 and 18 MPa (700 and 3,000 psi). Higher compressive strength might require more cement. But addition of more cement can affect the permeability of the walls. Indeed, properly constructed rammed earth endures for thousands of years, as many ancient structures that are still standing around the world demonstrate. In areas of high seismic activity, rammed-earth walls are reinforced with rebars. Adding cement to soil mixtures low in clay can also increase the load-bearing capacity of rammed-earth edifices. The United States Department of Agriculture observed in 1925 that rammed-earth structures endure indefinitely and can be constructed for less than two-thirds of the cost of standard frame houses. One significant benefit of rammed earth is its high thermal mass: like brick or concrete, it absorbs heat during the day and releases heat at night. This action moderates daily temperature variations and reduces the need for air conditioning and heating. In colder climates, rammed-earth walls can be insulated by inserting insulation such as styrofoam or rigid fibreglass panels within internal and external layers of rammed earth. Depending on the type and content of binder, it must also be protected from heavy rain and insulated with vapour barriers. Rammed earth can effectively regulate humidity if unclad walls containing clay are exposed to an internal space. Humidity is regulated between 40% and 60%. The material mass and clay content of rammed earth allows an edifice to breathe more than concrete edifices. This avoids problems of condensation and prevents significant loss of heat. Rammed-earth walls have the colour and texture of natural earth. Moisture-impermeable finishes, such as cement render, are not used by some people because they impair the ability of a wall to desorb moisture, which quality is necessary to preserve its strength. Blemishes can be repaired using the soil mixture as a plaster and sanded smooth.
The thickness varies widely based on region and code. It can be as little as 6 inches (150 mm) for non load-bearing walls and up to 24 inches (600 mm) for load-bearing walls. The thickness and density of rammed-earth walls make them suitable for soundproofing. They are also inherently fireproof, resistant to termite damage, and non-toxic.
Environmental effects and sustainability
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